Add amdgpu and mgag200 to the archiso_kms mkinitcpio hook

See https://wiki.archlinux.org/index.php/Kernel_mode_setting#Early_KMS_start .

archiso/initcpio/install/archiso_kms

 #!/bin/bash
 
 build() {
+    add_module "amdgpu"
     add_module "radeon"
     add_module "nouveau"
     add_module "i915"
+    add_module "mgag200"
     add_module "via-agp"
     add_module "sis-agp"
     add_module "intel-agp"